Afterburner for gas from gassification plant

ABSTRACT

The present invention relates to an afterburner for gas from a gasification plant. The afterburner provides an optimal mixture of combustible gas and air, permitting an optimal reaction between the O2 in the air and the gas and creating a mixture ratio that enables the load on a burner to be regulated without altering the mixture ratio between air and gas. This offers the possibility of optimal combustion even during regulation of the burner, over the whole of the relevant regulating  range. The result is extremely good combustion and reduced emissions of O2, CO and NOx.

The present invention relates to an afterburner for gas from a gasification plant. The afterburner provides an optimal mixture of combustible gas and air, permitting an optimal reaction between the O2 in the air and the gas and creating a mixture ratio that enables the load on a burner to be regulated without altering the mixture ratio between air and gas. This offers the possibility of optimal combustion even during regulation of the burner, over the whole of the relevant regulating range. The result is extremely good combustion and reduced emissions of O2, CO and NOx.

During combustion of gas it is important that air and gas should be mixed to form a homogeneous mass and that the combustible gas is permitted to react fully with the O2 content in the air. This creates an optimal mixture of combustible gas and O2 which is crucial for achieving clean, good and efficient combustion of the gas. This in turn provides a high level of utilisation of the combustible gas and a low level of emission of noxious gases and soot.

Furthermore, for a plant for combustion of gas, whether it stems from wood fuel or oil (atomized oil), it is important to be able to vary the load on the burner unit over an appropriate load range in order to obtain a flexible plant. Relevant examples are combustion of gas from wood for production of steam which in turn is employed in a steam turbine for production of electricity or combustion for heating oil which is circulated in a plant for heating and/or drying of, for example, wood. When the load on, for example, the electric generator increases, the energy supply to the steam turbine has to be increased and consequently the heating of the steam has to increase. This is accomplished through the supply of air and thereby also fuel. The air volume is traditionally regulated by regulating the air flow to the burner in step with the load. This causes the air velocity and turbulence in the mixing zone to be reduced correspondingly which in turn leads to a less efficient mixture of air and gas.

Amongst the known solutions are several for mixing combustible gas and air and a common solution is the supply of air in connection with a constriction or venturi where gas from smouldering wood pulp is mixed with air and combusted.

The fact which is particularly important and which forms the basis for the present invention is that the air is brought together with the combustible gas at high velocity and thereby with high turbulence. It is also important for the velocity of the air to be maintained. This is particularly important when the burner installation has to be regulated as indicated above and it is important for the velocity to be maintained over the entire regulating range. It will therefore be possible to regulate the volume of air in the same way as in the combustion process, but in the mixing phase the air velocity and turbulence are constant over the entire regulating range. In this way the good mixture of air and combustible gas is maintained and the reaction between the combustible gas and the air's O2 remains optimal at all regulating stages within the regulating range. This in turn leads to good, clean combustion and good utilisation of the calorific value in the combustible gas. This is also crucial for keeping down the costs per produced power unit.

On this basis, according to the present invention an afterburner is provided for mixing combustible gas and air, which afterburner comprises a substantially circular mixing chamber with open ends where the combustible gas is introduced into the mixing chamber at the first end. The afterburner is characterised in that air is added to the mixing chamber along the mixing chamber's circumference through one or more openings in the wall of the mixing chamber from an air supply chamber so that the combustible gas and the air are mixed in the mixing chamber and where the mixture of the combustible gas and the air are discharged from the other opening in the mixing chamber and where the air is introduced into the mixing chamber from the air supply chamber substantially tangentially to the interior of the mixing chamber and has a velocity generated by a fan in connection with the air supply chamber. In this way a swirl of air and combustible gas is created where the air spins in a rotating motion through the afterburner. According to the so-called spin rate, the angular velocity w multiplied by the radius of the mixing chamber will be constant. In a preferred embodiment of the invention the diameter D1 of the mixing chamber's air inlet and the diameter D2 of the mixing chamber's outlet are different and the diameter D1 of the mixing chamber's air inlet is preferably larger than the diameter D2 of the mixing chamber's outlet. Since the spin rate is constant, the angular velocity will increase when the diameter is reduced.

Furthermore, the air supply chamber may surround the part of the mixing chamber where the air inlet openings in the mixing chamber's walls are provided. This enables the air to be easily passed from the fan for supplying air to the afterburner according to the invention.

In a further embodiment an overflow chamber may be connected to the air supply chamber, which overflow chamber is provided with an outlet and a damper in connection with the outlet. By adjusting the damper the volume of air passing from the air supply chamber to the overflow chamber will be regulated. The volume of air which is not supplied to the mixing chamber will thereby be regulated since it bypasses the inlet. In a further embodiment the overflow chamber may surround the whole or parts of the mixing chamber and is connected to the air supply chamber. In order to regulate the air to the mixing chamber, the position of the damper can therefore be varied with the result that the damper varies the air flow out of the overflow chamber.

In an embodiment the connection between the air supply chamber and the overflow chamber is substantially at the openings between the air supply chamber and the mixing chamber.

In different embodiments a cone may be provided at the inlet to the mixing chamber, forcing the combustible gas out towards the air inlet openings in the mixing chamber and assisting in creating swirling in the mixing chamber. In a further embodiment thereof, in the mixing chamber at the inlet to the mixing chamber, a cone may be provided whose pointed end points towards the inlet to the chamber. At this cone, moreover, oil may be supplied which is atomized or is already atomized.

The ingoing air to the mixing chamber has a velocity direction which is substantially tangential and the combustible gas has a velocity vector which is substantially axial. The tangential velocity vector is determined by the air supply fan's capacity and pressure (combustion air). The axial velocity vector is determined by the area in the burner where the air flows, which in turn is determined by the ratio between D1 and D2 as indicated above. The resulting velocity vector has a direction with an axial and a tangential component. Through regulation of supplied air the resulting velocity vector will be altered by reducing the axial velocity while the tangential velocity is increased. The air velocity will therefore be varying with little variation and will be approximately constant over the regulating range, giving a higher velocity where the velocity would normally be reduced with known solutions, thereby producing the highly favourable combustion possibilities provided by the invention.

The invention is further explained with reference to the attached figures, in which:

FIG. 1 is a cross sectional view from the side of an embodiment of the invention with mixing chamber together with chambers for air inlet and overflow.

FIG. 2 illustrates the air inlet chamber in section A-A from FIG. 1.

FIG. 3 illustrates the overflow chamber in section B-B from FIG. 1.

FIG. 4 is a graphic presentation of air and gas velocity together with resulting velocity with full load on the burner.

FIG. 5 is a graphic presentation of air and gas velocity together with resulting velocity with regulated load on the burner.

FIGS. 6 and 7 illustrate alternative embodiments of the inlet to the afterburner according to the present invention.

FIG. 1 is a cross sectional view from the side of an afterburner according to the present invention with a mixing chamber 1, an inlet 2 for combustible gas and an outlet 3 for combustible gas mixed with air. Also illustrated is an air supply chamber 6 surrounding the mixing chamber with connections 4 to the mixing chamber 1. The air supplied to the air supply chamber 6 comes from a fan which gives the air a velocity and a pressure. An overflow chamber 7 is further illustrated where excess air can be discharged and this is controlled by a damper 9 (FIG. 3) at the outlet of the overflow chamber 7.

The combustible gas enters the mixing chamber through the opening 2 and air is supplied through the openings 4 from the air supply chamber 6. If the load on the burner is reduced, the supply of air is regulated by letting some air pass through the openings 5 and on out into the overflow chamber. This is regulated by the damper 9 (FIG. 3) in the overflow chamber.

It is further illustrated in FIG. 1 that the inlet for combustible gas has a diameter D1 while the outlet of mixed gas and air has a diameter D2. D2 is smaller than D1 and this difference gives increased velocity to the air axially through the mixing chamber.

Furthermore, in cross section A-A from FIG. 1, FIG. 2 is a view from below (from the inlet side) of an air supply chamber 6. It shows that this has an inlet where the air is supplied by a fan. Moreover it is apparent that the air rotates in the chamber 6 and is admitted to the mixing chamber through the openings 8 with the result that the air's direction is substantially tangential in the chamber 1 where the air meets the combustible gas and is mixed. The air here has a high velocity and high turbulence and the mixture with the combustible gas is highly effective and the gas essentially reacts fully with the air. This has been proved by means of experiments and the following measurements of O2, CO and NOx have been made showing that the combustion gives values that are bordering on theoretical without smoke being observed from the chimney. This applied, furthermore, over the whole load range.

Furthermore, in FIG. 3 the overflow chamber 7 is illustrated where excess air can escape instead of being mixed into the mixing chamber 1. It also shows that the chamber 7 has an outlet with a damper 9 which is adjusted in order to remove air from the mixing chamber 1. If the damper 9 is completely closed, all the air goes to the mixing chamber while if the damper 9 is fully open, a substantial part of the air goes outside the mixing chamber 1.

FIG. 4 further illustrates in a diagram the ratio between axial and tangential air velocity and the resulting air velocity and direction through the mixing chamber. FIG. 5 further illustrates a corresponding diagram where the axial velocity is reduced as a result of less air supply (a greater proportion to the overflow chamber 7). Since the velocity ratio is constant, the angle of the resultant flow (the vector) will be constant and the velocity will also be constant.

Furthermore, in FIGS. 6 and 7 alternative embodiments are illustrated where a cone 10 is mounted at the inlet 2 for combustible gas, forcing the gas out towards the peripheral edge of the chamber 1 where the gas meets the air (at high velocity) and is mixed. FIG. 7 further illustrates that the cone 11 may contain an outlet 12 for supplying oil which has been or is being atomized. 

1. An afterburner for mixing combustible gas and air, which afterburner comprises a substantially circular mixing chamber with open ends, an inlet and an outlet, where the combustible gas is introduced into the mixing chamber at the inlet, and where air is added to the mixing chamber along the mixing chamber's circumference through one or more openings in the wall of the mixing chamber from an air supply chamber so that the combustible gas and the air are mixed in the mixing chamber, and where the mixture of the combustible gas and the air is discharged from the outlet in the mixing chamber, and where the air is introduced into the mixing chamber from the air supply chamber substantially tangentially to the interior of the mixing chamber and has a velocity generated by a fan in connection with the air supply chamber, wherein an overflow chamber is connected to the air supply chamber, which overflow chamber is provided with an outlet and a damper in connection with the outlet.
 2. An afterburner according to claim 1, wherein a diameter D1 at the mixing chamber's inlet and a diameter D2 at the mixing chamber's outlet are different.
 3. An afterburner according to claim 2, wherein the diameter D1 at the mixing chamber's inlet is larger than the diameter D2 at the mixing chamber's outlet.
 4. An afterburner according to claim 1, wherein the air supply chamber surrounds the part of the mixing chamber where the air inlet openings in the mixing chamber's walls are provided.
 5. An afterburner according to claim 1, wherein the overflow chamber surrounds the whole or parts of the mixing chamber and is connected to the air supply chamber.
 6. An afterburner according to claim 1, wherein the damper's position can be varied with the result that the damper varies the airflow out of the overflow chamber.
 7. An afterburner according to claim 1, wherein the connection between the air supply chamber and the overflow chamber is substantially at the openings between the air supply chamber and the mixing chamber.
 8. An afterburner according to claim 1, wherein in the mixing chamber at the inlet to the mixing chamber, a cone is provided whose pointed end points towards the inlet of the mixing chamber, at which cone oil may be supplied. 